Romney responded by praising Trump for "an extraordinary ability to understand how our economy works and to create jobs" and for being "one of the few who has stood up to say China is cheating" in international trade.

It was unclear whether Trump's decision will have any impact on the Republican race. A Pew survey last month found that 64% of definite and likely GOP voters said an endorsement from the reality television star would make no difference to them.

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In the survey, 13% said it would make them more likely to back a candidate, while 20% said it would actually make them less likely.

"Endorsements rarely sway voters," Republican strategist Alex Castellanos said. But "the Trump endorsement undermines (Newt) Gingrich's argument that it is just the Washington establishment that is out to stop him and nominate Romney. Trump is the ultimate outsider."

Trump was not always so positive regarding Romney's record. During an interview on CNN's "State of the Union" in April, Trump criticized Romney for eliminating jobs while in the private sector.

"He'd buy companies. He'd close companies. He'd get rid of jobs," Trump told CNN Chief Political Correspondent Candy Crowley. "I've built a great company. My net worth is many many times Mitt Romney."

Asked what changed since then to bring his endorsement of Romney, Trump told CNN later Thursday that his past comments were "political talk."

"That was a long time prior to my getting to know him," Trump said of Romney. "But I have gotten to know him and he's a terrific guy. I don't know if he really comes out like he really is in person. He's a warm, smart, tough cookie and that's what this country needs. We need somebody that's tough, that will stop China and OPEC and all these other nations from just ripping us up. And i think he can do it."

Perhaps in reference to Trump's earlier comments, Romney said Thursday after thanking Trump for the endorsement, "I spent my life in the private sector, not quite as successful as this guy but successful nonetheless."

News of Trump's endorsement of the Republican front-runner came as the GOP candidates stumped for votes across Nevada and Romney continued to wrestle with fallout from a CNN interview in which the former Massachusetts governor said he wasn't "concerned about the very poor."

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"We have a safety net" for the very poor, Romney told CNN's Soledad O'Brien on Wednesday. "If it needs repair, I'll fix it. I'm not concerned about the very rich; they're doing just fine. I'm concerned about the very heart of America, the 90%, 95% of Americans right now who are struggling, and I'll continue to take that message across the nation."

Pressed by O'Brien, Romney noted that the poorest Americans have access to food stamps, Medicaid and housing vouchers.

"You can choose where to focus," he said. "You can focus on the rich; that's not my focus. You can focus on the very poor; that's not my focus. My focus is on middle-income Americans."

Romney later insisted that his words were taken out of context and reiterated the full context of the quote, which was meant to stress his focus on the middle class.

"You've got to take the whole sentence, (or else) it sounds very different," he said. "We have a safety net for the poor. ... If there are people that are falling through the cracks, I want to fix that."

The comment, however, sparked a new round of debate over an emerging narrative of Romney as a plutocrat unaware of the needs and concerns of ordinary Americans. It also evoked memories of previous controversial Romney statements, such as when he said he likes firing people who provide poor service.

Romney is "reinforcing the image of the national party that is elite (and) out of touch," said Mark McKinnon, a former campaign adviser to George W. Bush and John McCain. "That's why he's having such problems with independent voters. ... Unless the Republicans turn that around, they cannot win this election."

Gingrich, the former House speaker, has blasted Romney for the remark over the past two days, declaring Wednesday that he is "fed up with politicians in either party dividing Americans against each other."

On Thursday, Gingrich said that "we should care about the very poor -- unlike Governor Romney. But I believe that we should care differently than Barack Obama. Both Governor Romney and Barack Obama seem to believe that a 'safety net' is all the poor need. I don't believe that. What the poor need is a trampoline so they can spring up and quit being poor."

Regardless of the extent of Romney's verbal miscue, the former governor is now the clear Republican front-runner.

A national poll released Wednesday showed an immediate benefit for Romney, putting him in first place at 31% compared to 26% for Gingrich, 16% for Santorum and 11% for Paul.

The Gallup daily tracking poll had Gingrich ahead of Romney after the former speaker won the January 21 South Carolina primary.

Romney cemented his status as the GOP leader with an overwhelming victory in Tuesday's Florida primary. Romney won the primary with 46% of the vote, compared with 32% for Gingrich, 13% for Rick Santorum and 7% for Ron Paul, according to the Florida Department of State.

The victory gave Romney all 50 of Florida's convention delegates and, more important, new momentum heading into a series of caucuses and primaries building up to Super Tuesday on March 6, when 10 states will hold nominating contests.

Gingrich's campaign asked the Republican Party on Thursday to allocate the 50 Florida delegates on a proportional basis, rather than the winner-take-all format that gave them to Romney.

In a letter to the Republican National Committee, the Gingrich campaign argued that an RNC rule says early primaries and caucuses must award delegates proportionally. However, party rules say the issue can be addressed only at the August nominating convention in Tampa, Florida.

Santorum also has expressed support for Florida to change its system, but the Florida Republican Party chairman, Lenny Curry, said in a statement Thursday that won't happen.

"Florida was winner-take-all before election day, we were winner-take-all on election day, we will remain winner-take-all," Curry's statement said, adding: "It is a shame when the loser of a contest agrees to the rules before, then cries foul after losing."